With rapid development in mobile communications, radio terminal equipment represented by mobile telephones are springing into wide use.
The development owes greatly to advancement in high-frequency integrated circuit technology and development of smaller, lighter, and higher-performance antennas. As an example of such an antenna, a helical antenna produced by forming a helical conductor on an insulating rod is disclosed in Japanese Patent Laid-open Publication No. 10-65432 (1998). Although this antenna is being used as a substitute for a whip-type (rod-shaped) antenna and contributing to the provision of a smaller and lighter antenna, it is of a type used by being projected outward from the apparatus and not of a type mountable on a circuit board.
On the other hand, surface-mountable type antennas disclosed in Japanese Patent Publication No. 3011075 and Japanese Patent Laid-open Publication No. 9-64627 (1997) are mountable on a circuit board. Here, the antenna element is produced by laminating a plurality of dielectric sheets or dielectric substrates having conductive pattern formed thereon to provide a multiple-layered member and connecting the patterns with conductors through holes made in the sheet or board thereby forming a product with a modified helical shape. These antennas are complicated in structure and require a large number of component parts and further had problems with mechanical strength, electrical performance, and environment-resistive performance. The antenna disclosed in Japanese Patent Laid-open Publication No. 9-74309 (1997) improved the surface-mounted type antenna of Japanese Patent Laid-open Publication No. 9-64627 in terms of mechanical strength and environment-resistive performance and partly improved it in terms of electrical performance. The antennas disclosed in Japanese Patent Laid-open Publication Nos. 9-223908 and 9-232828 further improve the antenna in terms of electrical performance. The basic structure of these antennas is not greatly different from that of the aforesaid Japanese Patent Laid-open Publication No. 9-64627, i.e., these are similarly produced by laminating substrates with conductor patterns printed thereon and electrically connecting the patterns. Thus, they have problems of complexity of structure, multiplicity of components, and production of variations in antenna characteristics among individual antennas leading to the requirement of circuit adjustments for absorbing the variations, and hence poor productivity of the antennas.